1. Field of the Invention
The present invention relates to an air bag apparatus mounted in a vehicle in order to protect a on-board passenger by activating a bag body to be unfolded and inflated in the direction of the on-board passenger. More particularly, the present invention relates to an air bag apparatus fit for use as what is intended for a passenger seat or a steering wheel of an automobile.
Although a description will subsequently be given of an air bag apparatus which is mounted in an instrument panel and intended for a passenger seat by way of example, the present invention is also applicable to an air bag apparatus for any other outfit such as a steering wheel and a rear seat as long as the air bag apparatus is similar in construction.
2. Description of the Related Art
An air bag apparatus for a passenger seat generally comprises an inflator 12, a bag body 16 having gas vent holes 14 in such a folded-up condition that the gas emitted from the inflator 12 can flow into the bag body, and a cover body 18 for covering the bag body 16 in that folded-up condition. The cover body 18 is basically equipped with a hinge door portion (a casement door as illustrated) 20 which is opened when the bag body is initially unfolded and inflated (see FIGS. 1, 2). The bag body 16 is, as shown in FIG. 2, formed by sewing one sheet of strip-cloth-like main panel 22 and two sheets of planar eggplant-shaped side panels 24 and provided with a bag-mounting base portion 28 having bolt fitting holes 26 on the root portion side of the bag body. Reference numeral 13 in FIG. 1 denote a defuser can; 15, a bag retainer; and 17, a bag housing can.
As shown in FIG. 3, the bag body 16 may be a planar circle 16A in shape. In FIG. 3, reference numeral 14A denotes gas vent holes; 26A, bolt fitting holes; and 28A, a bag-mounting base portion.
The following four steps are followed when an air bag is activated.
First step: The inflator is ignited and the gas discharged from the inflator flows into the bag body. At this time, the bag body tries to become unfolded and inflated but the presence of the cover body on the front side in the inflating direction causes the unfolding and inflation of the bag body to be temporarily suppressed. Accordingly, the gas pressure in the bag body is sharply increased and there develops the force of thrusting up the cover body from the rear side (inside).
Second step: When the force of thrusting up the cover body from the inside exceeds a predetermined value, the hinge door portion is opened as a groove portion formed in the cover body is ruptured, and the bag body pops out of the cover body at high velocity. This is because the quantity of gas flowing into the bag body from the inflator reaches a peak with the absence of an obstacle for preventing the bag body from inflating.
Third step: After completion of the unfolding and inflation of the bag body, the quantity of gas flowing from the inflator exceeds the peak and enters a reducing stage. At this stage, the bag body comes into contact with the on-board passenger.
Fourth step: While pressing against the bag body out of inertia force, the on-board passenger receives counterforce, that is, deceleration (G) from the bag body. As the gas flows out of the vent holes, texture, seams and the like when the on-board passenger thus presses against the bag body, the deceleration received by the on-board passenger is lowered, whereupon the on-board passenger is restrained (protected).
It is desirable to suppress the inflation speed of the bag body toward the on-board passenger at the (second) stage at which the bag body pops out of the cover body at the aforesaid second stage.
Although it is considered possible to reduce the quantity of gas flowing into the bag body from the inflator in order to suppress the inflation speed, the time required to substantially complete the inflation of the bag body which is brought into contact with and used to protect the on-board passenger is also prolonged by a large margin and this is also undesirable.